Sub-phylum Urochordata: Definition, Characteristics, Classification, Examples

The sub-phylum Urochordata (Gr., uro, tail; chorde, cord) is a part of the phylum Chordata. The animals in this group are called sea squirts,or tunicates. These animals are very unique and show a high degree of diversity in their form, habit, and habitat.

The chordate characters of Urochordata are more noticeable during the larval stage.

Definition of Urochordata

Sub-phylum Urochordata represents the animals that have notochord only at their larval stage and can exhibit retrogressive metamorphosis, the formation of colonies, and the alternation of generations.

Subphylum Urochordata or Tunicata

Characteristics of Urochordata

  • The animals are strictly marine.
  • They may be sessile (Ascidiacea) or permanently pelagic (Thaliacea or Larvacea) in form.
  • Adults are unsegmented and mostly attached to a substratum.
  • The notochord is restricted to the caudal or tail region in the larval stage. Hence, the name of this group is given as Urochordata.
  • A dorsal hollow nerve cord, pharyngeal gill slits, and a prominent post-anal tail are also present in the larval stage.
  • The nerve cord extends from one end to the other.
  • The body of the adult is soft, and it is covered by a flexible tunic or test. Hence, these animals are commonly known as tunicates. The tunic is composed of the protein tunicin and cellulose.
  • There are two openings at the free end of the body. One is the mouth, and the other is the atriopore.
  • The proximal part of the alimentary canal is greatly expanded and forms a wide pharynx.
  • The circulatory system is open, and the heart is simple and tubular.
  • The pyloric gland, neural gland, and nephrocytes are found as the excretory organs.
  • Reproduction takes place both sexually and asexually.
  • These animals are mostly hermaphrodites. Fertilization occurs externally.
  • Asexual reproduction takes place through the buds formation.
  • Development shows retrogressive metamorphosis, which occurs through a free-swimming tadpole larva.
  • Presence of the alternation of generations in the life cycle.
  • Colony formation occurs in some Urochordates.

Examples of Urochordata

Ascidia, Salpa, Pyrosoma, Doliolum, Herdmania, Molgula, and Appendicularia.

Classification of Urochordata

Sub-phylum Urochordata is divided into three classes: Ascidiacea, Thaliacea, and Larvacea.

Here, the classification is adopted from Berril (1950). The outline of the classification is given below:

  • Class 1. Ascidiacea
    • Order 1. Enterogona (e.g., Ascidia)
    • Order 2. Pleurogona (e.g., Molgula)
  • Class 2. Thaliacea
    • Order 1. Pyrosomida (e.g., Pyrosoma)
    • Order 2. Doliolida (e.g., Doliolum)
    • Order 3. Salpida (e.g., Salpa)
  • Class 3. Larvacea
    • Order 1. Copelata (e.g., Appendicularia)

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